This patch series introduces a cgroup subsystem that utilizes the swsuspfreezer to freeze a group of tasks. It's immediately useful for batch jobmanagement scripts. It should also be useful in the future for implementingcontainer checkpoint/restart.

The freezer subsystem in the container filesystem defines a cgroup file namedfreezer.state. Reading freezer.state will return the current state of thecgroup. Writing "FROZEN" to the state file will freeze all tasks in thecgroup. Subsequently writing "RUNNING" will unfreeze the tasks in the cgroup.

# echo RUNNING > /containers/0/freezer.state # cat /containers/0/freezer.state RUNNINGAndrew, since I hear Rafael doesn't have time to review these (again) at thistime, please consider these patches for -mm.

Cheers, -Matt Helsley

Changes since v4:v5: Split out write_string as a separate patch for easier merging with trees lacking certain cgroup patches at the time. Checked use of task alloc lock for races with swsusp freeze/thaw -- looks safe because there are explicit barriers to handle freeze/thaw races for individual tasks, we explicitly handle partial group freezing, and partial group thawing should be resolved without changing swsusp's loop. Updated the patches to Linus' git tree as of approximately 7/31/2008. Added Pavel and Serge's Acked-by lines to Acked patches

v4 (Almost all of these changes are confined to patch 3): Reworked the series to use task_lock() instead of RCU. Reworked the series to use write_string() and read_seq_string() cgroup methods. Fixed the race Paul Menage identified. Fixed up check_if_frozen() to do more than just test the FROZEN flag. In some cases tasks could be stopped (T) and marked FREEZING. When that happens we can safely assume that it will be frozen immediately upon waking up in the kernel. Waiting for it to get marked with PF_FROZEN in order to transition to the FROZEN state would block unnecessarily. Removed freezer_ prefix from static functions in cgroup_freezer.c. Simplified STATE_ switch. Updated the locking comments.

v2 (roughly patches 3 and 5): Moved the "kill" file into a separate cgroup subsystem (signal) and it's own patch. Changed the name of the file from freezer.freeze to freezer.state. Switched from taking 1 and 0 as input to the strings "FROZEN" and "RUNNING", respectively. This helps keep the interface human-usable if/when we need to more states. Checked that stopped or interrupted is "frozen enough" Since try_to_freeze() is called upon wakeup of these tasks this should be fine. This idea comes from recent changes to the freezer. Checked that if (task == current) whilst freezing cgroup we're ok Fixed bug where -EBUSY would always be returned when freezing Added code to handle userspace retries for any remaining -EBUSY--